1. Field of the Invention
The present invention relates to an illuminator including a light-emitting device and fluorescent materials capable of radiating various color lights by converting wavelength of light emitted from the light-emitting device, and more particularly, it relates to an illuminator also having an optical transmission function of transmitting optical signals through space.
2. Description of the Background Art
Japanese Patent Laying-Open No. 2002-290335 discloses a space optical transmitter as an exemplary illuminator also having a space optical transmission function. This space optical transmitter includes a transmission-side device for energizing LEDs (light-emitting diodes) of red, green and blue serving as illumination light sources while modulating power waveforms supplied to the light sources in response to information and a receiving-side device for demodulating the information from the illumination light through photodetection means for receiving the illumination light from the transmission-side device.
According to such a space optical transmitter, the transmission-side device can be arranged on a ceiling thereby avoiding obstruction in optical transmission, and the facility having both functions of communication and illumination can be simplified by rendering the communication function and the illumination function in common.
The aforementioned space optical transmitter disclosed in Japanese Patent Laying-Open No. 2002-290335 must employ a plurality of LEDs for emitting at least three color lights of red, green and blue as the light sources for light to be transmitted, so as to also have the illumination function.
When wavelengths within the visible band are employed for transmission light in a situation where sunlight is present, the sunlight acts as background noise light against color lights to be transmitted, so that no sufficient S/N (signal-to-noise ratio) can be kept in optical transmission and hence no sufficient transmission rate can be achieved.
Japanese Patent Laying-Open No. 2002-290335 also suggests that the so-called WDM (wavelength division multiplex) communication can be enabled by introducing different signal data into three primary color lights of the space optical transmitter and transmitting the same. However, driving voltages for the respective LEDs for emitting lights of red, green and blue depend on energy band gaps of semiconductor emission layers included in the LEDs. In other words, the driving voltages for driving the LEDs for emitting lights of different colors are remarkably different from each other, so that it is necessary to provide the LEDs with different driving circuits designed to operate with the respective voltages.